Electrorheological fluids are colloidal suspensions whose electrorheological properties can be varied through the application of an external electric field. In particular, under the application of a field of the order of 1–2 kV/mm, an electrorheological fluid can exhibit a solid-like behavior, such as the ability to transmit shear stress. This transformation from liquid-like to solid-like behavior can be very fast, of the order of 1 to 10 ms, and is reversible when the electric field is removed.
Electrorheological fluids are of interest because potentially they can provide simple, quiet, and fast interfaces between electrical controls and mechanical systems. As such, they have a number of potential applications including automotive clutches, ABS brakes, shock absorption, vibration damping and micro-electric mechanical systems.
Most previous electrorheological fluids are based on the usage of micron-sized particles and on the large dielectric contrast between the particles and the fluid. A problem of this prior art is that the yield strength is too low for many practical applications, which results from large currents and breakdown. The yield strength of these electrorheological fluids is typically no more than 3 kPa at 1 kV/mm. The yield stress of the nanoparticle-based electrorheological fluids reaches up to 40 kPa when suitable promoters are added (U.S. patent application Ser. No. 10/243,668). However, there is much room for improvement of the performance of the electrorheological fluids by varying the parameters of the material components and the synthesis procedures.